Problem: Evaluate the following expression when $a = 1$ and $b = 1$. $7$ $a$ $ + 4$ $b$ $ - 9$
Explanation: Substitute $1$ for ${a}$ and $1$ for ${b}$ $ = 7{(1)} + 4{(1)} - 9 $ $ = 7 + 4 - 9 $ $ = 2$